Thursday, February 21, 2008

The forensics of light

With Photoshop, we are (in essence) playing with colour and light. We are bound to end up testifying at some point about what we've done with light - or stuck for a simple explanation in a sales meeting. We've talked about colour temperature before. How do we tell stories about light? What is light?

Light as a function of the EM spectrum: only a small section of the electromagnetic spectrum is visible to the human eye. We call this the visible color spectrum. We think of light in terms of three aspects: wavelength, frequency and amplitude. The wavelength distinguishes among different types of light. The frequency of a light wave determines its color. Amplitude refers to the brightness of a light wave.

Gamma rays have a frequency of 1018 Hz, (sorry for the problems with superscript) while radio waves have a frequency of 104 Hz. Green is at the center of this spectrum. Blue light waves have a higher frequency and are hardest to see - its the amplification of the blue channel in digital CCDs that leads to excessive noise. Red light waves have a lower frequency. Thinking of visible light with a frequency of about 1014 Hz: this means that our eyes receive just more than four hundred trillion light waves every second.

Frequency and wavelength are inversely related. As wavelength increases, the frequency decreases. Think of it this way, if there are a lot of little waves hitting the beach quickly, they could be said to have a short wavelength and a high frequency. If there are only a few big waves hitting the beach slowly, they would have a long wavelength and lower frequency.

The amplitude of a light wave is what our eyes perceive as brightness. The greater the amplitude of a light wave, the further the wave is displaced from the mid-line, and the brighter the light.

Start collecting stories about the processes in which you employ. Perhaps even create a database of these stories. Light, colour, noise, reflection, and etc can all be explained simply. Your database or list of simple explanations helps when preparing your testimony.

You've read in this blog and elsewhere that there tends to be more noise in the blue channel than the others. Seeing blue as a function of frequency helps you and your audience relate to the why - it has a higher frequency and is thus harder to see. Because it's harder to see, it needs to be amplified. This amplification can sometimes introduce noise into the blue channel. Different manufacturers will have varying success at this mechanical process, which is why some cameras have more noise than others.


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